It's not if you don't just take it literally and consider the context of the situation.
Zoro and Franky saying that back in Zou was basically just them telling Luffy to not worry and leave things to them in their own way, not that it would literally have to happen.
Like when Momo screamed I WANT TO TAKE DOWN KAIDO to express his frustrations and ask for Luffy's help no one was thinking OH SHIT MOMO WILL DEFEAT KAIDO.
But then Zoro expresses his desire to fight him and people take it as Zoro literally having a personal desire to cut Kaido's head off.
Zoro and Franky could of expressed their words of encouragement in ways that still have their individual flavor mixed in without it seeming like declarations, implying some sort of action will be taken to fulfill them. Also they should know that Luffy isn't a man that need words of encouragment, especially since he was already gun-ho about the plan.
Momo did say that he want to take down Kaido, but he knew that he was too weak, which was beating him up inside, knowing that he couldn't be of much help. That's why he requested to Shinobu to age him up, to help the raid in anyway. Even now he wants to become stronger to protect all of Wano from any threat. He's taking actions to make up to be able to fulfill such remarks in the future.
As for Zoro's remark of taking Kaido's head, well that was just a line that lead absolutely nowhere as he never contemplated on why he failed to even put down Kaido, so I'd actually consider that bad writing just as I would with Zoro and Franky's statements in Zou.
They don't have to fulfill their declaration, but they need to acknoledge that they have failed to do so, and will figure out how to avoid such a failure from happening again, which Momo has done a good job of doing. Same can't be said for the Luffy knobgobbler Zoro.